CNC and CMM machine tools are well known in the art and employed in virtually every field of manufacturing for tasks that include, for example, material cutting, material removal and inspection. Typically, these machine tools have one or more movable elements, such as a work table or tool spindle, with each movable element being movable along one or more orthogonal axes and/or about a rotational axis.
As is well known in the art, the quality of the output from a CNC machine or CMM is a highly complex function that is based in part on the accuracy and repeatability with which the movable elements of the machine tool are movably positioned. Those of even the most basic skill in the art will readily appreciate that the quality of the output is influenced by diverse other factors, such as the accuracy and repeatability of the fixturing that is employed to hold a tool bit or workpiece, but these other factors are outside the scope of the present application and as such, will not be discussed in detail herein.
One way in which accuracy and repeatability may be obtained is through relatively tight tolerancing of the various critical machine tool elements, such as ball screws and ways, that control the movement and/or positioning of a machine tool's movable element. Issues of wear aside, one drawback with this proposition concerns its economics; generally speaking, tolerances may only be tightened to a degree without unreasonable cost penalty, but thereafter substantial costs are incurred so that progression (i.e., the tightening of the tolerances) beyond a point is cost prohibitive. With that in mind, the various critical machine tool elements of most if not all CNC machines and CMM's used in manufacturing are not capable of positioning a machine tool's movable elements in an extremely accurate manner (i.e., on the order of one ten-thousandth of an inch) without some sort of electronic compensation.
Typically, a CNC machine or CMM utilizes a controller that includes two sets of registers for each axis on or about which a movable element may be moved. The first set of registers usually employs a single register for use in compensating for reversal error, which is also known as lost motion or backlash. Reversal error occurs when the movable element is moved in a first direction along or about the axis and thereafter moved in a second direction opposite the first. The second set is a series of registers that correspond to the position of the movable element along or about the axis at various predetermined points. The registers of the second set are intended to correct for errors in the positioning of the movable element at the various predetermined points over its full range of motion along or about the axis.
Various methodologies have been developed to quantify the accuracy and repeatability of a CNC machine or CMM. Generally, these standards have been developed by professional societies, such as the American Society of Mechanical Engineers (ASME), and standards organizations, such as the International Standard Organization (ISO), the VDI/DGQ German Standard Organization, the Association for Manufacturing Technology (AMT) (formerly the National Machine Tool Builders Association (NMTBA)) and the National Institute of Standards and Technology (NIST) (formerly the American National Standard Institute (ANSI)). Software tools that provide information on the formats of such standards are well known and commercially available.
While the details of these methodologies vary somewhat, they share analysis techniques wherein error is quantified as a band or range and wherein the values for the accuracy and repeatability of the machine tool are based on the width of the error band. I have found, however, that values for accuracy and repeatability calculated in this manner are too conservative to be of any meaningful value.
The plot illustrated in FIG. 1 shows the results of an accuracy and repeatability analysis (hereinafter capability study) on a CNC machine using one commercially available software tool. In this study, the error band is based on a spread of six standard deviations (i.e., ±3 standard deviations) about the mean of each test point, with the purported accuracy and reliability being based on the maximum width of the error band. In this example, the axis of the machine tool is stated to have an accuracy of 0.003850 inch and a repeatability of 0.003650 inch. Stated another way, the results of this capability study imply that if the movable element of the machine tool were told or commanded to move along the axis to a given point (X), the actual position of the movable element would be within a band of 0.003850 inch (i.e., X±0.001925 inch) and that successive repositioning of the movable element to the given point would place the movable element within a band of 0.003650 inch (i.e., [X±0.001925 inch]±0.001825 inch). Accordingly, one would ordinarily conclude that this axis of the CNC machine would only be useful in manufacturing if the tolerances on the part to be fabricated were larger than about ±0.004 inch.
As the data in the plot clearly illustrates, however, this conclusion is too conservative. Let us assume, for example, that we were to use this CNC machine to fabricate a part that is only two inches in length and that we position the part at the far left of the movable element. Since all of the data points from the capability study fall within an approximately ±0.0005 inch band in this region, the performance of this axis of the CNC machine under these circumstances will be considerably better than ±0.004 inch.
In view of the drawbacks of the known methodologies for quantifying the accuracy and repeatability of a CNC machine or CMM, as well as the common tendency of these methodologies to ignore the calibration process (i.e., the initializing or setting of the first and second registers which is discussed in detail, below), I had found no known method for programming or initializing the first and second sets of registers that produced accurate and repeatable results prior to developing my invention. The methodology most commonly utilized consisted of measuring the error in the positioning of a movable element along or about an axis at each of location designated in the second set of registers. This methodology appears to make use of the mechanic's conventional wisdom wherein the mechanic programs a given register in the second set to correct for measured error at a corresponding location of the movable element. For example, let us assume that the movable element is a rotary spindle that is movable in a generally vertical direction (Z axis) and that a given register in the second set corresponds to a positioning of the spindle at a location 0.50000 inch below a predetermined datum. When the spindle is told or commanded to move 0.50000 inch below the datum, the spindle does not move to that exact position—there is some error (let us assume for this example that the spindle moves to a point 0.50100 inch below the datum). Using his conventional wisdom, the mechanic corrects for the error in positioning by entering a corresponding offset into the given one of the second registers. In the example provided, the offset would be −0.00100 inch.
As this process is repeated for each of the registers in the second set, and as there are often times 50 or more (often times hundreds) registers in the second set for a single axis, the process is rather time consuming and costly (both in terms of the cost of the mechanic's time as well as in machine down-time where it cannot be used in a productive manner). Considering that many CNC machines and CMM's used in manufacturing are of the type having several movable elements that are collectively movable along or about several axes, the amount of time to calibrate a single machine tool can be upwards of several days.
With the machine tool thus calibrated, one would think that the movable elements of the machine tool would be capable of being moved with great accuracy and repeatability. Mechanics, however, have found that this is not the case; rather, they have observed that changes to the values in the registers of the second set, which are often times significant in magnitude, are nearly always necessary on each successive test to eliminate the positioning error at each given point.
From the foregoing, it is apparent that there remains a need in the art for an improved methodology by which a CNC machine or CMM may be calibrated and its accuracy and repeatability more accurately determined.